Stand density significantly influences functional trade-offs and synergies in plantation management. This study examined Robinia pseudoacacia L. plantations, measured trees, shrubs, herbs, litter, and soil across stand density gradients. Carbon sequestration, water conservation, biodiversity protection, and soil conservation functions were quantified for each stand density. Our study revealed that stand density negatively correlated with carbon sequestration and biodiversity conservation, but positively with water conservation. Soil conservation responded nonlinearly, declining initially before stabilizing with increasing stand density and soil depth. Soil conservation was identified as the dominant function in the study area. A trade-off existed between carbon sequestration and water conservation, while other functions exhibited synergies. Functional interrelationships and the comprehensive function index revealed prevalent trade-offs across most stand densities, with synergies observed only at 1250 and 1475 plants/ha. The comprehensive function index fluctuated with stand density, -initially decreasing, then increasing, followed by further fluctuations-with synergies correlated with lower indexes and trade-offs with higher indexes. The optimal stand density range for Robinia pseudoacacia plantations was 1368-1852 plants/ha. Random forest modeling was applied to predict optimal management densities. Stands with densities of 1575-1775 plants/ha required minimal management intervention. Lower-density stands required underplanting at varying intensities, while higher-density stands needed selective thinning at differentiated levels. The results of this study provide decision-making support for sustainable ecosystem management and a theoretical basis for future Robinia pseudoacacia plantation management.